Edit existing virtual machine's xml-file and start virtual machine with SPICE like follows.
This site's example has created a virtual machine without graphics,
so it's OK to change settings like follows,
but if you created virtual machine with a graphics, Remove "<graphics>***" and "<video>***" sections in xml file because qxl is used for graphics.

By the way, if you'd like to enable SPICE on initial creating of virtual machine, specify like follows.
Then, it's possible to install Systems with SPICE which requires GUI like Windows without installing Desktop Environment on KVM Host computer.